1. Field of the Invention
The present invention relates to oil-based drilling fluids for use in drilling wells in subterranean formations for the production of hydrocarbons. More particularly, the present invention relates to improved formulations for oil-based drilling fluids, especially invert emulsion drilling fluids, and to methods of using same, for enhanced shale stabilization.
2. Description of Relevant Art
Drilling fluids or drilling xe2x80x9cmudsxe2x80x9d are used in drilling oil and gas wells to serve several purposes, including lubricating the drill bit, removing cuttings from the wellbore, and providing stability to the well. The appropriate design of a drilling fluid for a particular well is a major factor in the economic success of drilling a well.
Generally, drilling fluids are either water-based or xe2x80x9coilxe2x80x9d (or non-aqueous) based. As used herein, the term xe2x80x9coil-basedxe2x80x9d shall be understood to include synthetic-based fluids and invert emulsion fluids as well as traditional oil-based fluids.
Invert emulsion drilling fluids are comprised of water dispersed in oil in the liquid phase. They are more cost-effective than water muds in a variety of situations, including, for example, without limitation, the situation of drilling through shale formations. Invert emulsion fluids can reduce or eliminate the water wetting of formation shales and thereby minimize or eliminate problems associated with sloughing shales, borehole collapse, gumbo clays and drilled solids contamination known to the petroleum industry.
An invert emulsion fluid has three phases: base fluid, water, and solids. Tiny droplets of water are dispersed in the base fluid to form a generally microscopically heterogeneous mixture of the base fluid and the water. Because emulsions are so composed of two immiscible liquids, a large amount of interfacial tension occurs at the water-droplet/base fluid interface. Thus, surface active agents or surfactants or emulsifiers are added to lower such interfacial tension to increase the stability of the emulsion.
Osmosis is the flow of a solvent (water) from a solution with a low salinity into a solution with a higher salinity through a membrane that is permeable to the solvent but not to the dissolved salt. The osmotic pressure that develops depends on the salinity of the two solutions; the greater the difference in salinity, the higher the osmotic pressure.
The salinity required in an invert emulsion fluid to avoid water transfer and the resulting swelling of formation clays (the water demand) depends on the depth and pore pressure of the shale and on the salinity of the water in the shale. When the water demand of the mud is equal to the water demand of the shale, the forces are in balance and the osmotic pressure to start water flow between the two is zero. Osmotic pressure sufficient to dehydrate shales (to cause water to flow from the formation into the mud) can be generated with salt solutions. Selecting a salt depends on formation type and its demand for water.
Calcium chloride has most commonly been the salt of choice because it offers a greater range of activity than other salts like sodium chloride. Despite its common use, calcium chloride nevertheless has become less preferred in recent years because of environmental concerns. More environmentally friendly substitutes are needed.
The present invention provides a method for drilling a wellbore in a subterranean formation, particularly or especially formations comprising shales, such that borehole erosion is substantially reduced, and borehole instability due to water absorption by the shales is effectively eliminated, as shale hydration is effectively eliminated. The method employs an invert emulsion, or an oil (or synthetic) based drilling fluid comprising an invert emulsion, which is believed to be stabilized by the method of the invention.
According to the method of the invention, the water activity of the formation and the invert emulsion is determined. A formate or acetate, and most preferably potassium formate, is added to the emulsion or the drilling fluid comprising the emulsion in a concentration such that the water activity of the emulsion (or drilling fluid) is about the same or less than the water activity of the formation. The water activity of the formation and of the emulsion (or drilling fluid) is monitored over time as the emulsion is used in drilling. Additional formate or acetate and most preferably potassium acetate is added to the drilling fluid to maintain the water activity of the fluid at about the same or lower levels than the water activity of the formation.